1. Field of the Invention
The present invention relates to phase shift mask blanks suitable for use in photolithography associated with the microfabrication of electronic products such as semiconductor integrated circuits, charge-coupled devices, color filters for liquid-crystal displays, and magnetic heads. The invention relates also to a method of manufacturing such phase shift mask blanks.
The invention relates most particularly to halftone phase shift mask blanks which can attenuate the intensity of exposure wavelength light with a phase shift film, and to a method of manufacturing such phase shift mask blanks.
2. Prior Art
The photomasks that are used in a broad range of applications, including the manufacture of semiconductor integrated circuits (IC) and large-scale integration (LSI) chips, are basically composed of a transparent substrate on which a light-shielding film made primarily of chromium has been formed in a given pattern. The market demand for ever higher levels of integration in semiconductor integrated circuits has led to a rapid reduction in the minimum feature size of photomask patterns. Such miniaturization has been achieved in part by the use of shorter wavelength exposure light.
However, while exposure using shorter wavelength light does improve resolution, it also has a number of undesirable effects, such as reducing the focal depth, lowering process stability and adversely impacting product yield.
One pattern transfer technique that has been effective for resolving such problems is phase shifting. This involves the use of a phase shift mask as the mask for transferring microscopic circuit patterns.
As shown in accompanying FIGS. 6A and 6B, phase shift masks (including halftone phase shift masks) are typically composed of a substrate 1 on which a phase shift film 2 has been patterned. The mask has both exposed substrate areas (first light-transmitting areas) 1a on which there is no phase shift film, and phase shifters (second light-transmitting areas) 2a that form a pattern region on the mask. The phase shift mask improves contrast of the transferred image by providing, as shown in FIG. 6B, a phase difference of 180 degrees between light passing through the pattern region and light passing through the non-pattern region, and utilizing the destructive interference of light at the boundary regions of the pattern to set the light intensity in the areas of interference to zero. The use of phase shifting also makes it possible to increase the focal depth at the necessary resolution. Hence, compared with a conventional mask having an ordinary exposure pattern, such as one composed of chromium film, a phase shift mask can improve resolution and increase the margin of the exposure process.
For practical purposes, such phase shift masks can be broadly categorized, according to the light-transmitting characteristics of the phase shifter, as either completely transmitting phase shift masks or halftone phase shift masks. Completely transmitting phase shift masks are masks in which the phase shifter has the same light transmittance as the substrate, and which are thus transparent to light at the exposure wavelength. In halftone phase shift masks, the phase shifter has a light transmittance that ranges from about several percent to several tens of percent the transmittance of exposed substrate areas.
Accompanying FIG. 1 shows the basic structure of a halftone phase shift mask blank, and FIG. 2 shows the basic structure of a halftone phase shift mask. The halftone phase shift mask blank shown in FIG. 1 is composed of a substrate 1 which is transparent to the exposure light and on which a halftone phase shift film 2 has been formed. The halftone phase shift mask shown in FIG. 2 is composed of halftone phase shifters 2a which have been obtained by patterning the phase shift film 2 and form the pattern regions of the mask, and exposed substrate areas 1a on which there is no phase shift film. The arrangement of the two types of areas defines the mask pattern.
Exposure light that has passed through a phase shifter 2a is phase-shifted relative to exposure light that has passed through an exposed substrate area 1a (see FIGS. 6A and 6B). The transmittance of the phase shifter 2a is selected such that exposure light which has passed through the phase shifter 2a has too low an intensity to sensitize the resist on the substrate to which the pattern is being transferred. Accordingly, the phase shifter 2a functions to substantially shield out the exposure light.
Halftone phase shift masks of the above type include halftone phase shift masks having a simple, single-layer construction. Single-layer halftone phase shift masks known to the art include those described in JP-A 7-140635 which have a phase shifting film composed of molybdenum silicide oxide (MoSiO) or molybdenum silicide oxide nitride (MoSiON).
Such phase shift masks may be manufactured by using a lithographic process to pattern a phase shift mask blank. The lithographic process typically involves applying a resist onto the phase shift mask blank, sensitizing desired areas of the resist with an electron beam or ultraviolet light, carrying out development to expose the surface of the phase shift film, then etching desired areas of the phase shift film using the patterned resist film as the mask so as to expose the substrate. The resist film is then stripped, giving the finished phase shift mask.
The phase shift mask is provided with a phase shift pattern by a dry etching operation in which the phase shift film is etched using the resist pattern formed on the transparent substrate as the mask. However, the phase shift film has a poor resistance to pre-treatment such as cleaning in the phase shift mask production process, and to cleaning fluids such as sulfuric acid used for such purposes. As a result, the optical constants of the phase shift film sometimes change during phase shift mask production.